Unearthing Clues to Martian Fossils

Unearthing clues to Martian fossils

The hunt for signs of ancient life on Mars
leads scientists to Mono Lake, CA

Above: Eerie
looking mineral towers called tufa rise out of Mono Lake, in
central California.
Some scientists think that strange precipitates in Mono Lake
hold important clues to the search for fossils of ancient life on Mars. Photo Credit: Tony
Phillips.
View 360 deg. IPIX Images of Mono Lake

June 11, 1999: If, while innocently enjoying this article,
you were unexpectedly
transported to the surface of Mars, three things would happen before you could finish reading:

First you would die, either from asphyxiation
or hypothermia. Mars's carbon dioxide atmosphere is
100 times less dense than Earth's
and the average surface temperature is -60 deg C.
The exact cause of death would depend on the season, the
time of day (Martian temperatures fluctuate as much as
100 degrees from dawn to dusk), and
the latitude of your surprise landing site.

Next you would begin
to dry out. There is no liquid water on
the surface of Mars
and little, if any, water vapor in the atmosphere. Your lifeless
body would become dessicated like an Egyptian mummy.

Finally, not that it would matter terribly, you would
contract a very nasty sunburn. The Red Planet's rarefied atmosphere
does a poor job blocking UV rays from the sun (there is no protective
ozone layer in the atmosphere). Radiation levels
are so intense that they probably sterilize the uppermost layers
of Martian soil.

Undoubtedly present-day Mars is not a congenial place for
life as we know it, but it may have been friendlier in the distant
past.
There is growing evidence to support a view of ancient Mars
as a remarkably Earth-like planet:
Between 3 and 4 billion years ago
liquid
water flowed
in channels and collected in lakes and ponds
all over the Red Planet. There may have even been
an ocean.
The surface temperature was a balmy 0o C or above
to allow liquid water. To allow that to happen, the atmosphere
must have been a lot denser than it is today.
A dynamic molten core gave rise to a global
magnetic field that protected Mars from the ravages of the solar
wind and powered tectonic activity
in the Martian crust. Hot
springs were likely commonplace. Billowing volcanoes resupplied a dense Martian
atmosphere with greenhouse gases needed to sustain a warm and
wet climate.

The reality of this picture is somewhat controversial, but if it is true,
it seems likely to many scientists that early
Mars could have teemed with simple forms of life.

"Microbial communities developed on early Earth in less than a billion years,
so it's plausible that simple organisms also developed on an early
wet and warm Mars," says Dr Jack Farmer, a geobiologist at Arizona
State University. "Current conditions on the martian surface
are hostile to life, but there might be a fossil
record of ancient microorganisms if we look in the right places."

Farmer (formerly of NASA/Ames) along with his collaborators at ASU, is a pioneer in the new scientific discipline called
exopaleontology -- the search for signs of primeval life on other
planets.

"Mars may harbor the best preserved rocks in the solar system," he
continued. "For example, the Allan Hills meteorite [an ancient potato-sized
rock from Mars that
crashed into Antarctica 13,000 years ago] is nearly 4.6 billion
years old. The fossil record on Mars might go all the way back
to the earliest history of the planet."

Farmer says he wouldn't mind visiting Mars to prospect for fossils
in person,
but an unmanned probe is likely to be the first exopalentologist
on the
Red Planet. Where should a Mars lander set down to seek out
the elusive fossil record? The answer to that question may be found here
on Earth in an other-worldly place called Mono Lake.

Mono Lake in California is nearly 700,000 years old, making it
one of the oldest lakes in North America. Throughout its long
existence, salts and minerals
have washed into the lake from Eastern Sierra streams, but there
is no outlet. Fresh water
evaporating leaves behind salts and
minerals so that now Mono Lake is about 2 1/2 times as salty
and 80 times as alkaline as the ocean. Swimmers in the lake
find that they literally cannot sink (dissolved carbonates,
chlorides and sulfates make
floating easy) but their
skin does tend to bleach and burn in the alkaline water.

Although Mono Lake is an extreme environment for life, it hosts
a thriving ecosystem. There are no fish,
but the lake supports trillions of brine shrimp (which feed vast
numbers of nesting and migrating birds) and a bizarre variety
of scuba-diving alkaline flies. It is also brimming with
microorganisms such as diatoms, cyanobacteria and filamentious
algae.

"The geology of
the Mono Basin reminds me of many old Martian lake beds," says Farmer.
"Take Gusev Crater for example. It's a basin on Mars formed
by an impact more than 3.5 billion years ago.
Water flowed in through channels in a huge canyon called
Ma'adim Vallis, but there was no outlet. It was
an evaporative lake site."

There is almost certainly no life in places like Gusev Crater today.
All
the ancient ponds and lakes on Mars are now bone dry and scorched by
solar UV radiation. Nevertheless, there could be fossils of
life forms that
thrived billions of years ago, and a curious geological
feature of Mono Lake may be telling us where and how to look for them.

At first glance the most striking aspect of Mono Lake are the
weird
mineral spires called tufa, a type of freshwater limestone.
They are formed
when calcium-rich spring water bubbles up through the
alkaline lake, which is rich in bicarbonate. The calcium
and bicarbonate combine, precipitating out as limestone.
Tufa towers only grow while underwater, but at Mono Lake they can be seen
towering as much as 12 feet above the surface. That's because
the lake level has been lowered in recent years
to supply water to Los Angeles, 360 miles to the south.

"Whenever you have minerals that precipitate rapidly as they do around the
springs in
Mono Lake, microorganisms become entombed," says Farmer. "The fossils
of soft-bodied microbes formed by this process
could be preserved for billions of years."

Farmer has spent many years studying the tufa at Mono Lake
as an analog of carbonate deposits that might one
day be discovered on Mars.

"There are lots of microfossils here and there in the tufa,
formed where
the rapid precipitation of carbonates captured
microorganisms," continued Farmer. "I've seen larval casings of
alkaline flies and cyanobacteria fossils, also things that look like
algae (simple multicellular plants).
I haven't yet found any fossils of
brine shrimp, but I'm still looking."

"In thin sections of tufa
I've also found clumps of decayed organic material called kerogen,
which may contain chemofossil signatures. Chemofossils
are the chemicals produced by the breakdown of cell walls. For example,
Mono Lake diatoms have a hard shell with an organic coating
that protects them from the alkaline water. When they die, the
coating dissolves and so does the diatom. All that's left of this organic
material is trace
chemicals. It is possible to relate such products to
specific organisms like
diatoms or algae, but its not always easy.
You have to become a Sherlock Holmes and piece together what the
community must have been like from clues (both chemical and fossil)
that are preserved."

"In evaporative basins, there's a lot of variation in
chemistry from basin to basin, and throughout
the history of the lake," Farmer continued. "What's beautiful
about Mono
Lake is that we have an active system of tufa-formation and
mineral precipitation. Other paleo lake basins in Western North America are
now dry
because the climate has changed and evaporation now dominates inflow."

In Search of Mono Lake -- on Mars?

Finding microfossils on Mars won't be easy, even if life
once existed there.
After all
Mars is a big planet and fossils are not likely to
be found just
anywhere.
No one knows for sure, but Farmer and collaborators think
that a good starting point might be evaporative basins
with carbonate deposits where microbial fossils could be entombed,
in other words, places that were once like Mono Lake.

The chemical mixture in an evaporative basin
depends on what kinds
of rocks are in the vicinity. When water flows into a lake, it
flows over rocks and dissolves minerals and ions such as sodium,
chloride ions,
pottasium, calcium -- all the salts commonly found
in the Western Salt Lakes. In an evaporative
basin the salts and minerals become concentrated, and the
lake naturally becomes alkaline with ph > 9. The detailed
chemical balance depends on the details
of the terrain.
This general picture is true on both Mars and Earth.

"Compared to Earth, Mars has a much different set of source rocks,"
explains Farmer.
"On Mars the crust is more like the ocean floor on Earth,
featuring basalts, iron, magnesium, and silicate-poor rocks.
Rocks in the Mono Basin are enriched in silica, sodium and
potassium. Because water was less abundant, it took longer
to build up briney water on Mars through evaporation. But the
waters there would be richer in calcium, magnesium, and iron.
In spite of these chemical differences, the basic picture
is still the same: rapid precipitation of minerals would
have been an important process in these ancient martian basins, and
if microorganisms were there, their fossils would have been entombed."

Phil Christensen, one of Farmer's collaborators, is using the Thermal
Emission Spectrograph (TES) on Mars Global Surveyor to
search out places on Mars with tufa-like
carbonate formations in evaporated lake beds.
Carbonates have specific kinds of absorption
features in mid-infrared spectra that should be easy to
identify. Unfortunately, the resolution of the TES is only
3 km/pixel, which would make smaller carbonate deposits like
those at Mono Lake difficult to detect.
In March 2001 an Arizona State University instrument called
THEMIS
(Thermal Emission Imaging System) is scheduled
for launch on NASA's Mars Surveyor 2001 orbiter. With a spatial
resolution of 100m per pixel, the ASU spectrometer could easily
detect the signature of carbonate deposits at the scale of the
Mono Lake tufas.

"I'm optimistic," concludes Farmer. "Eventually
I believe we're going to find carbonate deposits on Mars -- places
that remind us of
Mono Lake -- and when we do we'll have strong
arguments for a landing site for exobiology. It's just
a matter of time."
Related LinksRelated Stories:

The Red Planet in 3D --
New data from Mars Global Surveyor reveal the topography of Mars better than many continental regions on Earth.
May 27, 1999 NASA
NASA Science News

Appendix: Where did all the water go?

There may have once been ponds and lakes on Mars, but
they're dry now. Physical conditions on the surface of Mars,
namely low atmospheric pressure and low temperature, conspire
to make liquid water unstable. The average atmospheric pressure
on Mars is only about 6 millibars compared to the Earth's average
pressure of 1013 millibars. The average surface temperature on
Mars is about -60 deg C compared to the Earth's 15 deg C.
At certain locations and times on Mars, when the air pressure
is high enough and the temperature is above freezing (greater
than 0 deg C), liquid water is theoretically possible; but the
rate of evaporation would be so great that liquid water (if
it were present) would rapidly vaporize.

Nevertheless, there is widespread
evidence of dried-up valleys and channels thought to
have been eroded by liquid water.
Many Martian outflow channels strongly resemble flood
channels on Eath, like
those in eastern Washington in the USA.
On Mars they may have formed when groundwater or subsurface slush
was catastrophically brought to the surface, perhaps triggered
by large impacts or "marsquakes".
On the other hand, geological studies of the valley networks suggest that
these must have been gradually eroded
by running water: some show morphology suggesting formation by groundwater sapping (i.e. when a
river is fed by a spring and the valley grows by headward erosion); others seem to have been produced
by precipitation runoff. The valley networks are almost
completely (but not quite) restricted to ancient
upper highlands, dated as 3.5 to 4.0 billion years old from the quantity of impact craters, so it is
postulated that environmental conditions on Mars must have been conducive to liquid water at this time.

Right:
High resolution Mars Global Surveyor images were combined with Viking Orbiter color data to produce this
stunning, detailed view of a Martian canyon's edge. The area pictured is about 6 miles wide and represents a tiny part of the
northern edge of the canyon Valles Marineris, whose total length is about 2,500 miles. Details 20 to 30 feet across can be
seen in the high resolution data.
What
processes caused the well-defined layers in the steep canyon walls? In the Grand Canyon on planet Earth, sedimentary processes have resulted in spectacular
rock layers. But similar layers of rock in canyons of the Hawaiian Islands were created by volcanoes. Regardless of the origin
of layering on Mars, its extent suggests that early Mars was geologically active and complex.
More information.

The upper limit on the present amount of water on the martian surface is
800,000 to
1.2 million cubic miles (3.2 to 4.7 million cubic kilometers), or about 1.5 times the
amount of ice covering Greenland. If both caps are composed completely of
water, the combined volumes are equivalent to a global layer 66 to 100 feet (22
to 33 meters) deep, about
one-third the minimum volume of a proposed ancient ocean on Mars.